Oligosaccharide transport: pumping waste from the ER into lysosomes

N-glycans play important roles during the folding and secretion of glycoproteins. Surprisingly, during the N-glycosylation of glycoproteins, considerable amounts of unconjugated polymannose-type oligosaccharides ('free OS') are generated. Although free oligosaccharides have no known function in mammalian cells, a sophisticated cellular machinery enables them to be cleared from the endoplasmic reticulum (ER) into the cytosol and then re-enter the endomembrane system at the level of the lysosome. One possible function of this pathway is to stop free OS from interfering with the carbohydrate-dependent aspects of glycoprotein folding and transport along the secretory pathway.